Integrated circuit technology is finding many applications in the field of engine control, particularly with regard to controlling automobile engines. One of the engine parameters which often must be sensed by an engine control system is the rotational speed of the engine crankshaft. One known method for detecting the crankshaft speed is to cause the crankshaft to vary a magnetic field surrounding a sensor coil so as to generate an A.C. signal in the coil. An appropriate interface circuit may be used to detect the oscillations of the A.C. signal about a reference potential. The rotational speed of the crankshaft can then be determined since the frequency of the generated A.C. signal is proportional to the crankshaft speed.
In such engine control systems, large voltage transients having magnitudes from 100 to 200 volts are not uncommon. These large voltage transients can be harmful to the interface circuit coupled to the crankshaft speed sensor unless proper safeguards are employed.
Furthermore, noise signals often present in the environment of an engine control system can falsely trigger the interface circuit used to detect transitions of the A.C. signal. The introduction of hysteresis in switching circuits is known in the art for reducing the effect of noise signals and improving the noise margin of switching circuits. For example, a regenerative comparator or Schmitt trigger is shown by Strauss, Wave Generation and Shaping, 1970 McGraw-Hill Book Company, pp. 445-447.
Another consideration in constructing circuitry for detecting transitions of the A.C. signal about a reference potential is that the output waveform generated by the inerface circuit should be a symmetrical square wave, i.e., an output pulse with a fifty percent (50%) duty cycle. The reason for this requirement is that tachometer circuits, used to convert the frequency of the output waveform generated by the interface circuit into a D.C. voltage proportional to the crankshaft speed, often require an input pulse having a fifty percent (50%) duty cycle for maximum efficiency.
Further considerations in selecting an interface circuit for detecting transitions of the A.C. signal about a reference potential are suitability for operation with a single supply voltage as opposed to the need for both positive and negative supply voltages, and suitability for fabrication as an integrated circuit without requiring large amounts of chip area.
The difficulty in providing an interface circuit which is responsive to a ground referenced A.C. signal and which provides the advantages of hysteresis-type switching action while maintaining a symmetrical output waveform is that symmetric threshold levels (or trip points) must be provided above and below ground potential. Many prior art threshold circuits employ differential comparators wherein the input waveform is compared to one or more threshold levels. However, in applications where the threshold levels can be both above and below ground potential, these prior art differential comparator circuits require either both positive and negative power supplies or, if the circuit is to operate with a single supply voltage, extra circuit components for performing a voltage level-shifting function. Therefore, those skilled in the art should appreciate that an interface circuit responsive to a ground-referenced signal which provides a symmetric output waveform, which provides hysteresis-type switching action, and which may be fabricated as a highly dense integrated structure that requires a single power supply is a significant improvement over the prior art.